1. Field of the Invention
The present invention relates generally to methods for the fabrication of composite materials. More particularly, the present invention relates to a method for forming rigid composite preforms composed of reinforcement strands which have been served with a thermoplastic thread.
Composite materials are generally defined as a mixture or combination of two or more elements that differ in form and are essentially insoluble in one another. The present invention is concerned with strand-reinforced composites where a plurality of elongate reinforcement strands are embedded in a bulk matrix. Typically, the reinforcement strands are prepared from fibers or filaments having a high tensile strength, while the bulk matrix is an organic resin. The resulting composite materials thus possess the high strength of the reinforcement strands and are amenable to shaping by properly forming the matrix while in the softened, low viscosity condition. A wide variety of reinforcement materials have been used, typified by glass fibers, carbon and graphite fibers, silicon carbide fibers, metal fibers, aluminum oxide fibers, and the like. The matrix may be organic or inorganic, including organic resins, such plastic resins, epoxy resins, and inorganic materials, such as silicon carbide glass or metal mixtures.
Articles may be formed from composite materials by a number of techniques. Two dimensional sheets may be formed by first weaving the reinforcement strands into a flat sheet, followed by impregnation of the sheet with the desired matrix. The sheets may also be stacked as a prepreg to a desired thickness. They may also be stacked prior to impregnation by a technique referred to as hand lay-up, and subsequently impregnated. Three-dimensional articles may be formed by filament winding, where the elongate reinforcement strands are placed around a mandril having the desired shape with other yarns placed in the axial and radial directions. More complex multiple-dimensional shapes may also be formed on commercially available weaving machines.
The shaped fabric prior to impregnation with the organic matrix is usually referred to as the preform or prepreg. In some cases, the individual reinforcement strands will have been at least partially impregnated with a desired organic resin prior to the shaping operation, but the resulting article will still be referred to as a preform or prepreg since final impregnation and curing are required before the composite article is completed.
In order to obtain precisely sized and shaped composite articles from two-dimensional, three-dimensional, and multiple-dimensional weaves, it is necessary that the preforms be held in shape during the weaving and subsequent impregnation process. In some cases, this is achieved by leaving the preform on the shaping mandril during the impregnation step. In many cases, however, it is desired to remove the preform from the mandril prior to impregnation. Often, for example, the preform may be stored and transported from one location to another prior to the impregnation step. In these cases, it is necessary that the preform be sufficiently rigid to maintain its shape and preform yarn configuration. Moreover, handling flow of liquid during impregnation, curing shrinkage and expansion, and pyrolysis shrinkage, can also cause yarn and layer separation and deformation.
For the above reasons, it is desirable to provide methods for forming rigid preforms for use in fabricating composite articles. In particular, it is desirable that the preforms be made rigid without substantially affecting the yarn fiber volume or other properties of the resulting composite and that such rigidization be accomplished in an economical and efficient manner.
2. Description of the Relevant Art
U.S. Pat. No. 3,994,762 to Wrzesien et al. discloses the preparation of three-dimensional preforms using a needle punching technique. The preforms are pre-sized by impregnation with a thermosetting or thermoplastic material. U.S. Pat. No. 4,394,467 to Edelman discloses the coating of carbon fibers with a polyamic acid oligomer to facilitate holding the resulting preform together during a subsequent matrix impregnation step. U.S. Pat. No. 4,519,290 to Inman et al. discloses a machine for braiding reinforcement fibers in a composite material product. U.S. Pat. No. 3,779,851 relates to introduction of thermosetting plastics and/or thermoplastics to form prepreg plates which are later bonded together and cured. U.S. Pat. No. 3,728,423 relates to introduction of a thermosetting plastic to a fabric precursor which is subsequently molded at low temperature. The thermosetting plastic is not set, however, until the structure is carbonized and graphitized. U.S. Pat. No. 4,140,832 relates to prerigidization by employing particular carbon fibers having from 40 to 90% by weight mesophase by heating in an oxidizing environment. Finally, U.S. Pat. No. 3,914,395 discloses a multiple graphitization process where the matrix binder is initially converted to a continuous amorphous carbon matrix which holds the fibers in place. U.S. Pat. Nos. 4,152,381 and 4,320,079 are also of interest.